The present invention relates to a warning tone transmitter for application in underground mining, with a piezoelectric horn element with two input connections for the application of a supply voltage at a determined frequency and with a circuit arrangement fed from a supply current to generate the supply voltage.
In the course of the mining of the raw materials to be mined, such as coal, the advancing support frames finding application in underground mining are regularly withdrawn, moved forward in the direction of the mining face and then set in place again. It is important in this process that no mining personnel are present in the danger area, whereby various safety measures ensure that during the withdrawing, advancing and setting process of one or more advancing support frames nobody is injured.
One of these safety measures, which are provided on all support frames and are coupled to and activated by the advancing support frame controllers, are warning tones, which are activated before the start of the withdrawing process and generate a sufficiently long and loud warning tone, so as to alert miners working in the neighbourhood of the advancing support frame immediately before the withdrawing process to the danger resulting from this.
The previously proposed warning tone transmitters, which have previously been used for this have a piezoelectric horn element with two connections to which a supply voltage is fed, whereby the supply voltage has a frequency which corresponds to the tone generated by the horn. Since the loudest possible warning noise must be generated by the warning tone transmitter, it is endeavoured to bring the frequency of the supply voltage into agreement with the resonant frequency of the horn element in its assembled condition, since in the resonant case the sound volume obtainable from the warning tone transmitter is the greatest possible.
The aim of driving the known warning tone generators with their resonant frequency has in practice not been achievable. Since not only do the piezoelectric horn elements differ somewhat in their resonant frequency, but the resonant frequency of the horn element in its assembled condition depends also on the assembly conditions such as for instance the torque applied to screw fasteners or similar and the spatial environment of the warning tone transmitter also has a not inconsiderable influence on the resonant frequency, the sound volume at which the known warning tone transmitters which the known warning tone transmitters generate their warning tone is generally lower than would theoretically be possible. A further disadvantage of the known warning tone transmitter is that the controller of the support frame has no possibility of establishing whether a warning tone is actually being generated by the horn.
It is an aim of the present invention to produce a warning tone transmitter of the construction mentioned in the opening paragraph of the present specification, which can be driven in a simple manner at the resonant frequency, and thereby at its greatest possible sound volume, and which makes it possible to give the controller of the support frame an confirmation signal on the successful triggering of the horn signal.
Accordingly the present invention is directed to a warning tone transmitter as described in the opening paragraph of the present specification, in which the piezoelectric horn element has an additional output connection, which on the feeding of the horn element with the supply voltage generates an answering voltage signal which is taken to the circuit arrangement, and which is a value for the sound volume of the warning tone transmitter.
Using the third connection provided on the horn element it is possible to generate an actual signal, with whose help the calibration of the circuit arrangement is easily possible such that the voltage generated by it has a frequency which corresponds to the resonant frequency of the warning tone transmitter. The present invention makes use of the knowledge that the answering signal on the output connection, which is induced by the oscillation of the piezoelectric horn element and which consequently has a frequency which corresponds to the oscillatory frequency of the piezoelement and thus the frequency of the supply voltage, will assume a maximum value of voltage, when the horn is driven in resonance. It is only thus necessary for the calibration of the warning tone transmitter to vary the frequency of the supply voltage and at the same time to observe the answering signal and to adjust the supply voltage frequency to a value at which the signal delivered by the answering signal is a maximum. Such a measurement and calibration can be effected with very simple means. The answering voltage signal is in any case only generated when the piezoelement is actually oscillating and therefore generates a warning tone; the lack of an answering signal thus indicates that the warning tone transmitter has not generated the intended warning tone and the command for withdrawal and advancing of the support frame is not issued.
Advantageously the circuit arrangement has a self calibration arrangement for the supply voltage frequency, the circuit arrangement thus itself calculates the frequency at which the output voltage signal is a maximum. Such a self calibration can be effected by comparison measurements, preferably the self calibration device to work on successive approximations. In this procedure the required store for intermediate storage of the measured values is relatively lightly loaded and a fast and very precise calibration is achieved.
Preferably the circuit arrangement has a frequency generator to generate and change the supply voltage frequency. In a preferred embodiment the horn element is arranged in a preferably cylindrical horn casing and retained by a securing element which, for instance, can comprise a screw insert. It is also possible to use an O-ring or a circlip as the securing element. Thus the assembly of the horn element in the horn casing is possible in a fast and reliable in production.
Advantageously the horn element has a carrier plate with a piezoelectric membrane arranged upon it with a feedback pin arranged at or on it onto which the output connector is joined. The feedback pin oscillates in common with the piezoelectric membrane, when it is fed with a supply voltage of determined frequency. As a result of this oscillation the feedback pin generates a voltage at the output connector whose frequency corresponds to the oscillating frequency of the membrane and whose magnitude varies with the amplitude of the membrane and thus with the sound volume of the horn.
Preferably circuit arrangement has control electronics in the form of a microcontroller, which has outputs for the supply voltage signals and inputs for the line voltage and for the answering signal. Preferably the circuit arrangement has a rectifier circuit connected between the control electronics and the output connector, which rectifies the answering signal, so that it can be more readily processed by the control electronics. In a preferred embodiment the supply voltage signals from the control electronics are amplified by means of amplifying circuits before they are taken to the input connectors, so that the control electronics itself has only a low power consumption.
Advantageously the horn element and the circuit arrangement are arranged in a common horn casing, which then following its installation in the place provided for it on the advancing support frame or similar only requires to be connected to the power supply and to the actuating connection for the horn.